A Global Positioning System (GPS) is a satellite-based navigation system. There are two carrier frequencies commonly used by satellites, one is at 1575.42 MHz, the L1 channel, and other is at 1227.6 MHz, the L2 channel. Most commercial receivers only use the L1 channel. There are also two pseudorandom noise (PRN) codes used, the precise or protected (P) code and the course acquisition (C/A) code. The P code is classified and only used by military receivers. The C/A code is a sequence of zeros and ones and is unique for each satellite. Each zero or one is known as a “chip”. The C/A code is 1023 chips long, and it is broadcasted at 1.023 Mega-chips per second, i.e., the duration of the C/A code lasts one millisecond. Thus, it should be appreciated by those skilled in the art that the word “chip” may be regarded as a measurement unit of a data length or a time length. The property of C/A codes is that they have the best cross-correlation characteristic. The cross-correlation between any two C/A codes is much lower than auto-correlation of each C/A code individually. The C/A code is public and used by all receivers. Navigation data are also a sequence of zeros and ones at a rate of 50 bits per second, which carry satellite orbital position information. The navigation data and the C/A code are modulated by the carrier signal to form a GPS signal.
A GPS receiver can generate position coordinates by extracting and decoding the navigation data in GPS signals. A set of data collected by the GPS receiver usually contain GPS signals from several satellites. GPS signals from different satellites travel through different channels and are shifted in frequency due to the relative receiver-satellite motion. This is the so-called Doppler shift. Usually, each GPS signal has a different C/A code with a different code phase and a different Doppler shift, and the GPS receiver simultaneously processes the GPS signals from several channels. The code phase as used herein indicates a beginning point of the C/A code. Since the exact Doppler shift is unknown, a possible range of the Doppler frequencies should be searched. Also, to identify visible satellites, all possible C/A codes which are unique to each satellite should be searched. Therefore, to acquire the GPS signals from the visible satellites, the GPS receiver traditionally conducts a two dimensional search process for each received GPS signal, checking each C/A code with every possible code phase on every possible Doppler frequency. The acquired GPS signals may then be used to generate the position coordinates.
FIG. 1 illustrates a prior art block diagram of a GPS receiver 100. In general, the GPS receiver 100 includes a radio frequency (RF) front end module 101 and a base-band signal processing module 103. A GPS signal from a satellite is received by an antenna 102. Through a RF tuner 104 and a frequency synthesizer 105, the GPS signal is converted from a radio frequency signal to a signal with a desired output frequency. Then, an analog-to-digital converter (ADC) 106 digitizes the converted signal at a predetermined sampling frequency. The digitized signal is an intermediate frequency (IF) signal. The IF signal is then sent to the base-band signal processing module 103.
The base-band signal processing module 103 extracts navigation data from the IF signal and then calculates the location of the GPS receiver 100 based on the navigation data. In order to extract the navigation data, it is necessary to remove the carrier signal and the C/A code through an acquisition module 110 and a tracking module 112. The GPS signal is acquired by correlation in the acquisition module 110. The correlation is implemented by multiplying the IF signal with a local C/A code and a local carrier signal to obtain a product and then integrating the product over the GPS signal duration to obtain a correlation result. To acquire the GPS signal, the acquisition module 110 repeats the correlation on a two dimensional space by changing a code phase of the local C/A code and a Doppler frequency of the local carrier signal until the correlation result reaches a peak value. Once the GPS signal is acquired, the C/A code phase and the Doppler frequency are coarsely estimated and passed to the tracking module 112 for the tracking initialization. The tracking module 112 then removes the carrier signal and the C/A code in the GPS signal by adjusting itself to match the carrier signal and the C/A code. Information such as the navigation data is obtained at the tracking module 112. The GPS receiver 100 then decodes the navigation data to obtain satellite orbital position information through a post processing module 114, and the satellite orbital position information is then passed to a position calculation module 116 for calculation of the GPS receiver's position.
Ideal scenario for applications of such conventional GPS receivers is a clear view of sky so that line-of-sight GPS signals can reach the GPS antenna. However, due to the widespread interest in GPS technology, many applications require the GPS receivers to realize fast and precise positioning in some environments such as indoors and downtown, where GPS signals are greatly weakened and attenuated. In this situation, an assisted-GPS (AGPS) approach is implemented to improve the ability of GPS receivers in such environments.
FIG. 2 illustrates a prior art architecture of an AGPS. The exemplary AGPS architecture includes an AGPS server 201 with a reference receiver 207 which has line-of-sight views of available satellites. The reference receiver 207 continuously tracks the available satellites and logs satellite tracking information, such as satellite almanac, ephemeris, approximate user position and a time stamp. The AGPS server 201 collects the satellite tracking information from the reference receiver 207. The satellite tracking information makes it possible to predict satellite orbit and clock information for many days into the future. So the AGPS sever 201 generates assistance data for acquisition and track of incoming GPS signals and sends the assistance data to a base station 203. The base station 203 transmits the assistance data to a GPS receiver 205. The GPS receiver 205 may significantly improves time-to-first-fix and sensitivity by using the assistance data.
Despite the advantages offered by the assistance data, use of conventional approaches for GPS signal acquisition in a conventional GPS receiver does not adequately address the indoors and downtown challenge. Conventionally, due to the navigation data bits, which have a phase transition in 20 ms, the GPS signal acquisition module coherently correlates blocks of GPS signal spanning anywhere from 1 to 20 millisecond in duration. The results are then combined non-coherently, that is, the squares or magnitudes of block correlation functions are summed. This non-coherent combination results in “squaring loss,” reducing sensitivity. Therefore, the GPS receiver requires substantial improvements in sensitivity in the indoors and downtown environments. Thus, it is desirous to have an apparatus and method that significantly enhances the sensitivity of the GPS receiver and it is to such apparatus and method the present invention is primarily directed.